A conventional slip motor comprises a rotor having windings therearound. Induced potential in the windings is directed to the load through an electric brush and a slip ring. By varying the impedance in the circuit of the rotor, overcurrent in the windings of the rotor can be prevented, thereby preventing the motor from being damaged at low speeds. However, this means that the efficiency of such motors is low. Additionally, it is difficult to adjust and release the surplus energy produced by the motor.
The speed variation of a commutator motor is obtained by change of the rotary magnetic field with appropriate frequency produced by changing the position of the winding led to the AC power supply. The respons of this type of motor is somewhat erratic because of the appearance of sparks in the commutator, and the construction of the motor becomes complicated in order to eliminate such sparks.